Tales of volcanoes and El-Nino southern oscillations with the oxygen isotope anomaly of sulfate aerosol

Abstract

The ability of sulfate aerosols to reflect solar radiation and simultaneously act as cloud condensation nuclei renders them central players in the global climate system. The oxidation of S(IV) compounds and their transport as stable S(VI) in the Earth's system are intricately linked to planetary scale processes, and precise characterization of the overall process requires a detailed understanding of the linkage between climate dynamics and the chemistry leading to the product sulfate. This paper reports a high-resolution, 22-y (1980-2002) record of the oxygen-triple isotopic composition of sulfate (SO4) aerosols retrieved from a snow pit at the South Pole. Observed variation in the O-isotopic anomaly of SO4 aerosol is linked to the ozone variation in the tropical upper troposphere/lower stratosphere via the Ozone El-Niño Southern Oscillations (ENSO) Index (OEI). Higher (17)O values (3.3‰, 4.5‰, and 4.2‰) were observed during the three largest ENSO events of the past 2 decades. Volcanic events inject significant quantities of SO4 aerosol into the stratosphere, which are known to affect ENSO strength by modulating stratospheric ozone levels (OEI = 6 and (17)O = 3.3‰, OEI = 11 and (17)O = 4.5‰) and normal oxidative pathways. Our high-resolution data indicated that (17)O of sulfate aerosols can record extreme phases of naturally occurring climate cycles, such as ENSOs, which couple variations in the ozone levels in the atmosphere and the hydrosphere via temperature driven changes in relative humidity levels. A longer term, higher resolution oxygen-triple isotope analysis of sulfate aerosols from ice cores, encompassing more ENSO periods, is required to reconstruct paleo-ENSO events and paleotropical ozone variations.

Keywords: Cerro Hudson; El-Chichón; Intertropical Convergence Zone; Pinatubo.

Fig. 1.

(A) Oxygen isotopic composition (brown squares) and sulfate concentration profile (green diamonds) of composite aerosol sulfate samples extracted from the snow pit (6 m high) at the South Pole, Antarctica. The increase in sulfate concentration due to El-Chichón and Pinatubo + Cerro Hudson is also shown. UE, unknown event. (B) Comparison of oxygen isotope anomaly (red lines) and OEI (blue lines) obtained by Ziemke et al. (65) from the deseasonalized trend in total O₃ column measured at the equatorial Eastern and Western Pacific, an El-Niño region. Violet bars indicate three major ENSO events: ENSO-I (1982–1983), ENSO-II (1991–1992), and ENSO-III (1997–1998). A moderate event, ENSO-IV (1986–1987), is also shown. (The scale of ENSO events is defined by the National Oceanic and Atmospheric Administration and is available at www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml).

Fig. 2.

Four-isotope plot shows Δ¹⁷O and δ¹⁸O of sulfate aerosols extracted from the snow pit at the South Pole. A weak observed correlation indicates mixing of various sulfates from different sources. Red and green rectangles display variation in δ¹⁸O and Δ¹⁷O sources of sulfate. The blue rectangle is an oxidation source with stratospheric (Strat.) OH/HO₂ radicals (28, 39). The green rectangle shows the range of pure hydrogen peroxide oxidation. The red square denotes the value of atmospheric (Atm.) oxygen. Max., maximum. Primary sulfate produced during fossil fuel combustion at high temperature has been shown to possess δ¹⁸O values close to the atmospheric oxygen (51); however, sulfate produced during biomass burning showed a range of δ¹⁸O values (50) depending on biomass type. The observed dataset reflects the range of processes contributing to the observed oxygen isotopic composition of sulfate.

Fig. 3.

Concentration profile (1977–2003) of nss sulfate aerosol and MSA extracted from the snow pit samples at the South Pole. Peaks 1, 2, and 4 represent Pinatubo, Cerro Hudson, and El-Chichón volcanic sulfates, respectively. Peak 3 represents an unknown event.

Fig. 4.

Schematic depicts transport and transformation of sulfur species into the stratosphere and deposition of aged sulfate aerosol in the ice at the South Pole. The red-shaded area indicates a significant contribution of SO₂ and SO₄ aerosols to the SSA in the lower stratosphere, whereas the gray-shaded region represents carbonyl sulfide (OCS) photolysis and contribution to the SSA. The blue area sandwiched between these layers represents the ozone layer. Although O₃ production is maximum in the tropics, it is transported to the poles, as shown by the dynamics of the stratosphere with magenta lines. SSA, stratospheric sulfate aerosols; UT-LS exchange, air mass exchange at midlatitude between the upper troposphere and lower stratosphere.